I think I understand what you're going for but you must have meant something different? A single resistor isn't a crossover.
Besides that, if you meant a capacitor for protection purposes only, you would want a very large microfadarad value that would not impact any of the possible crossover points inside of the usable bandwidth. You're basically looking for something that will block low frequency content below 200-400Hz but without influencing stopband so that phase tracking is still good once the active points and slopes are applied. A cap offering "protection" at a frequency of 6kHz would destroy all of that and simply limit him to a 6dB slope at 6kHz.
In reality the protection capacitor just isn't necessary unless he's using a ribbon tweeter. Better off saving his money and avoiding the trouble of bad sound.